1. Field of the Invention
The present invention relates to a preliminary dispersion used in an ink receiving layer coating solution for forming an ink receiving layer of an ink jet recording medium, a fine particle dispersion including the preliminary dispersion dispersed further finely therein for use in an ink receiving layer coating solution, and an ink receiving layer coating solution including a binder, etc. added to the dispersion, and also to an ink jet recording medium having an ink receiving layer formed using the ink receiving layer coating solution.
2. Description of the Related Art
As the information technology (IT) industry has advanced rapidly in recent years, various information processing systems have been developed, and recording methods and recording devices suitable for the information processing systems have also been developed and practically used. Among the recording methods, an ink jet recording method is used widely not only in offices but also in homes because various kinds of recording materials can be used in recording, and the hardware (device) used therein is relatively inexpensive, compact and very quiet.
Along with higher resolutions attained by ink jet printers in recent years, various recording materials have been developed to realize photograph-like high-quality prints, etc. From the viewpoint of providing the recording materials with commercial value, it is required that the materials are excellent in ink absorptivity and in storage of images after printing, that is, the images are free of discoloration in long term storage and the ink does not smear thus preventing deterioration of the quality of the images.
An ink jet recording sheet provided with an ink receiving layer having a 3-dimensional structure with a high void volume, formed from fine inorganic pigment particles such as fumed silica and a water-soluble resin such as polyvinyl alcohol, is disclosed as a sheet with improved ink absorption (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 10-203006, 10-217601 and 11-20306), by which an image of high resolution can be formed.
The ink receiving layer is formed by coating a support with an ink receiving layer coating solution, and this coating solution is usually prepared by preparing a preliminary dispersion consisting of a kneaded product of fine inorganic pigment particles, then dispersing the preliminary dispersion further finely to form a fine inorganic pigment particle dispersion, and adding a binder, a mordant, a crosslinking agent, etc. to the fine inorganic pigment particle dispersion to prepare an ink receiving layer coating solution.
As one kind of the inorganic pigment, fumed silica consists of very fine particles, and thus the interaction among the particles is strong, and when fumed silica is added to and mixed with a dispersing medium such as water, the resulting dispersion has a pH of 4 to 5 and simultaneously exhibits thixotropic properties so that after being mixed and dispersed, the dispersion has a high viscosity and is poor in handlability. When the preliminary dispersion of high viscosity is further finely dispersed, the viscosity of the resulting dispersion also tends to be high. Accordingly, when the fumed silica is used as an inorganic pigment added to the ink receiving layer, it is not easy to prepare the preliminary dispersion and the dispersion.
Accordingly, there are known a method of producing a fine silica dispersion, which includes dispersing fumed silica in water to prepare a preliminary silica dispersion, then adding a boric acid compound and cationic polymer P-1 (with an I/O value of 4.3) thereto to adjust pH thereof to 5 to 8, and dispersing the dispersion further finely (paragraphs 0043 to 0045 of JP-A No. 2002-79741), and a method of producing a fine silica dispersion, which includes adding water, boric acid and borax to fumed silica, kneading and dispersing the mixture at pH 5 to 8 to prepare a preliminary silica dispersion, adding cationic polymer P-1 thereto, and dispersing the dispersion further finely (paragraphs 0046 to 0049 of JP-A No. 2002-79741).
However, when an image is recorded on an ink jet recording sheet prepared by using the preliminary silica dispersion or the fine silica dispersion, there is a problem of blurring, and in the above method using borax, sodium is contained in the ink receiving layer, and thus when an image is recorded thereon, there arises a problem of deterioration due to further blurring.
JP-A No. 2000-211235 (paragraphs 0056 to 0057) discloses using a mixed solvent of water and ethanol and adding a cationic polymer having an I/O value of 2.5 to prepare a preliminary silica dispersion. Further, JP-A No. 2002-52811 discloses adding a cationic polymer having an I/O value of 2.4 or less to prepare a preliminary silica dispersion. However, a preliminary silica dispersion of low viscosity cannot be obtained by using only the alcohol or only the cationic polymer having an I/O value of 2.4 or less.